The ability to provide long-distance wireless communication in dangerous and/or obstructed environments, for example in underground mines, is essential for both efficient operation and safety. In some applications, such as coal mining, communications systems are required by regulations for post-accident communications. In these mining operations, transmitting communication signals through earth, water and/or rock, for example, in addition to the large amounts of steel and concrete used in associated structures, severely impacts the effective range of the communications signal. This is especially true with systems transmitting radio frequency signals through, for example, conventional electric dipole antennas.
Magnetic communications systems (MCSs) that provide low-frequency magnetic signals offer an increased ability to transmit through these materials. For example, a MCS may be used to provide emergency communication between a site deep within an underground mine and a second distant site, either on the surface or also within the mine. These systems typically operate with relatively weak signals and severe interference as a result of above-described impedances and interferences, both natural and manmade. This interference limits the ability of the MCS receiver to detect and demodulate the underlying signal, limiting the system's effective range as a result.
In the field of signal processing, conventional methods to reduce impulsive interference include median filtering applied to recorded impulse-contaminated data. However, this filtering tends to distort the signal in impulse-free regions, in addition to not adequately removing the impulses. Median filters may be more advantageously applied when impulses contaminate a single isolated sample. However, impulses occupy multiple consecutive samples, as they represent the net response of the antenna, receive channel, and digital demodulator to very short interference pulses impinging on the antenna.
Other methods, such as impulse excision followed by extrapolation, also produce unsatisfactory results as the desired communications signal is typically dominated by other superimposed noise signals in addition to the impulses. Thus, extrapolation operates primarily on noise and ignores the underlying signal.
Improved systems and methods to eliminate or reduce these interferences are desired.